280 Multiple Alleles 



TABLE 10 

 Phenotypic Effects of Alleles at the a Locus in Maize 

 (Modified from Rhoades in Cold Spring Harbor Symposium.) 



QUESTIONS AND PROBLEMS 



1. If two Nicotiana plants of the constitution s^s'^ and sV are crossed 

 reciprocally, into what classes would the offspring of each cross segregate? 



2. If each class of the offspring of s^s^ X s^s^ were crossed with each 

 class of the offspring from the reciprocal cross, what percentage of the 

 crosses would be sterile? 



3. Diagram a pistil of an s^s^ plant. On the stigma place pollen from 

 sh^, s^s^, and s^s'^ plants. Draw the pollen tubes as they would appear 

 after several hours, showing which ones would have grown longer. 



4. Suppose that genes A and a are completely linked with self-sterilit}^ 

 alleles. What ratios would be obtained from the following crosses: 

 s^A I s^a X s^A / s%; s^A / s'^a X s^a / s^a; s^a / s^a X s^A / s^a; s^a / s-a 

 X s^a J ^A1 What ratios would be obtained if the self-sterility alleles and 

 the Aa genes were linked with 20 per cent crossing over? 



5. In Nejnesia strumosa, a self -sterile species, the following crosses 

 were made and results obtained {bu = buff flowers ; Bu = orange flowers) : 



Cross 

 Bubu X Bubu 

 Bubu X Bubu 

 bubu X Bubu 

 bubu X Bubu 



Orange 

 21 

 10 

 16 

 36 



Buff 



23 



10 







3 



